Oxidation products of fused 2-hetarylimidazole derivatives

Oxidation of 5-(1-methyl-1H-benzimidazol-2-yl)thiophene-, and -selenophene-2-carbaldehydes with potassium dichromate in 20% aqueous sulfuric acid afforded the corresponding carboxylic acids. Analogous reaction with 5-(1-methyl-1H-benzimidazol-2-yl)furan-2-carbaldehyde led to the formation of 1-methyl-1H-benzimidazole as a result of decarboxylation of the primary oxidation product and subsequent decomposition of the furan ring. Probable factors responsible for instability of 5-(1H-benzimidazol-2-yl)-hetarene-2-carboxylic acids were considered. The oxidation of 2-furylnaphtho2,3-d]- and 2-hetarylphenanthro-9,10-d]imidazoles gave, respectively, an anthraquinone analog and 6,7-quinones. π-Electron-rich heterocycles in 2-furyl- and 2-pyrrolylphenanthro9,10-d]imidazoles were oxidized completely, being replaced by hydrogen.